Community Ecology - Seneca Valley School Districtcompetition driving one to extinction. Resource Partitioning. Species that live together partition the available resources, reducing

515

25Community Ecology

Concept Outline

25.1 Interactions among competing species shapeecological niches.

The Realized Niche. Interspecific interactions often limitthe portion of their niche that they can actually use.Gause and the Principle of Competitive Exclusion. Notwo species can occupy the same niche indefinitely withoutcompetition driving one to extinction.Resource Partitioning. Species that live togetherpartition the available resources, reducing competition.Detecting Interspecific Competition. Experiments areoften the best way to detect competition, but they havetheir limitations.

25.2 Predators and their prey coevolve.

Predation and Prey Populations. Predators can limit thesize of populations and sometimes even eliminate a speciesfrom a community.Plant Defenses against Herbivores. Plants use chemicalsto defend themselves against animals trying to eat them.Animal Defenses against Predators. Animals defendthemselves with camouflage, chemicals, and stings.Mimicry. Sometimes a species copies the appearance ofanother protected one.

25.3 Evolution sometimes fosters cooperation.

Coevolution and Symbiosis. Organisms have evolvedmany adjustments and accommodations to living together.Commensalism. Some organisms use others, neitherhurting or helping their benefactors.Mutualism. Often species interact in ways that benefitboth.Parasitism. Sometimes one organism serves as the foodsupply of another much smaller one.Interactions among Ecological Processes. Multipleprocesses may occur simultaneously within a community.

25.4 Ecological succession may increase speciesrichness.

Succession. Communities change through time.The Role of Disturbance. Disturbances can disruptsuccessional change. In some cases, moderate amounts ofdisturbance increase species diversity.

A ll the organisms that live together in a place are calleda community. The myriad species that inhabit a tropi-

cal rain forest are a community. Indeed, every inhabitedplace on earth supports its own particular array of organ-isms. Over time, the different species have made manycomplex adjustments to community living (figure 25.1),evolving together and forging relationships that give thecommunity its character and stability. Both competitionand cooperation have played key roles; in this chapter, wewill look at these and other factors in community ecology.

FIGURE 25.1Communities involve interactions between disparate groups.This clownfish is one of the few species that can nestle safelyamong the stinging tentacles of the sea anemone—a classicexample of a symbiotic relationship.

Processes other than competition can also restrict therealized niche of a species. For example, a plant, the St.John’s-wort, was introduced and became widespread inopen rangeland habitats in California until a specializedbeetle was introduced to control it. Populations of the plantquickly decreased and it is now only found in shady siteswhere the beetle cannot thrive. In this case, the presence ofa predator limits the realized niche of a plant.

In some cases, the absence of another species leads to asmaller realized niche. For example, many North Americanplants depend on the American honeybee for pollination.The honeybee’s population is currently declining for a vari-ety of reasons. Conservationists are concerned that if thehoneybee disappears from some habitats, the niche of theseplant species will decrease or even disappear entirely. Inthis case, then, the absence—rather than the presence—ofanother species will be cause of a relatively small realizedniche.

A niche may be defined as the way in which an organismutilizes its environment. Interspecific interactions maycause a species’ realized niche to be smaller than itsfundamental niche. If resources are limiting, twospecies normally cannot occupy the same nicheindefinitely.

516 Part VII Ecology and Behavior

The Realized NicheEach organism in an ecosystem confronts the challenge ofsurvival in a different way. The niche an organism occupiesis the sum total of all the ways it utilizes the resources of itsenvironment. A niche may be described in terms of spaceutilization, food consumption, temperature range, appro-priate conditions for mating, requirements for moisture,and other factors. Niche is not synonymous with habitat,the place where an organism lives. Habitat is a place, niche apattern of living.

Sometimes species are not able to occupy their entireniche because of the presence or absence of other species.Species can interact with each other in a number of ways,and these interactions can either have positive or negativeeffects. One type of interaction is interspecific competi-tion, which occurs when two species attempt to utilize thesame resource when there is not enough of the resource tosatisfy both. Fighting over resources is referred to as inter-ference competition; consuming shared resources iscalled exploitative competition.

The entire niche that a species is capable of using,based on its physiological requirements and resourceneeds, is called the fundamental niche. The actual nichethe species occupies is called its realized niche. Becauseof interspecific interactions, the realized niche of aspecies may be considerably smaller than its fundamentalniche.

In a classic study, J. H. Connell of the University ofCalifornia, Santa Barbara investigated competitive inter-actions between two species of barnacles that grow to-gether on rocks along the coast of Scotland. Of the twospecies Connell studied, Chthamalus stellatus lives in shal-lower water, where tidal action often exposed it to air,and Semibalanus balanoides (called Balanus balanoides priorto 1995) lives lower down, where it is rarely exposed tothe atmosphere (figure 25.2). In the deeper zone, Semi-balanus could always outcompete Chthamalus by crowdingit off the rocks, undercutting it, and replacing it evenwhere it had begun to grow, an example of interferencecompetition. When Connell removed Semibalanus fromthe area, however, Chthamalus was easily able to occupythe deeper zone, indicating that no physiological or othergeneral obstacles prevented it from becoming establishedthere. In contrast, Semibalanus could not survive in theshallow-water habitats where Chthamalus normally oc-curs; it evidently does not have the special adaptationsthat allow Chthamalus to occupy this zone. Thus, the fun-damental niche of the barnacle Chthamalus included bothshallow and deeper zones, but its realized niche wasmuch narrower because Chthamalus was outcompeted bySemibalanus in parts of its fundamental niche. By con-trast, the realized and fundamental niches of Semibalanusappear to be identical.

25.1 Interactions among competing species shape ecological niches.

Fundamentalniches

Realizedniches

Chthamalus

Semibalanus

FIGURE 25.2Competition among two species of barnacles limits niche use.Chthamalus can live in both deep and shallow zones (itsfundamental niche), but Semibalanus forces Chthamalus out of thepart of its fundamental niche that overlaps the realized niche ofSemibalanus.

Gause and the Principle ofCompetitive ExclusionIn classic experiments carried out in 1934 and 1935, Russ-ian ecologist G. F. Gause studied competition among threespecies of Paramecium, a tiny protist. All three species grewwell alone in culture tubes, preying on bacteria and yeaststhat fed on oatmeal suspended in the culture fluid (figure25.3a). However, when Gause grew P. aurelia together withP. caudatum in the same culture tube, the numbers of P.caudatum always declined to extinction, leaving P. aureliathe only survivor (figure 25.3b). Why? Gause found P. au-relia was able to grow six times faster than its competitor P.caudatum because it was able to better utilize the limitedavailable resources, an example of exploitative competition.

From experiments such as this, Gause formulated whatis now called the principle of competitive exclusion.This principle states that if two species are competing for alimited resource, the species that uses the resource more ef-ficiently will eventually eliminate the other locally—no twospecies with the same niche can coexist when resources arelimiting.

Niche Overlap

In a revealing experiment, Gause challenged Parameciumcaudatum—the defeated species in his earlier experiments—with a third species, P. bursaria. Because he expected thesetwo species to also compete for the limited bacterial foodsupply, Gause thought one would win out, as had happenedin his previous experiments. But that’s not what happened.Instead, both species survived in the culture tubes; the

paramecia found a way to divide the food resources. Howdid they do it? In the upper part of the culture tubes, wherethe oxygen concentration and bacterial density were high,P. caudatum dominated because it was better able to feed onbacteria. However, in the lower part of the tubes, the loweroxygen concentration favored the growth of a different po-tential food, yeast, and P. bursaria was better able to eat thisfood. The fundamental niche of each species was the wholeculture tube, but the realized niche of each species was onlya portion of the tube. Because the niches of the two speciesdid not overlap too much, both species were able to sur-vive. However, competition did have a negative effect onthe participants (figure 25.3c). When grown without a com-petitor, both species reached densities three times greaterthan when they were grown with a competitor.

Competitive Exclusion

Gause’s principle of competitive exclusion can be restatedto say that no two species can occupy the same niche indefinitelywhen resources are limiting. Certainly species can and do co-exist while competing for some of the same resources. Nev-ertheless, Gause’s theory predicts that when two speciescoexist on a long-term basis, either resources must not belimited or their niches will always differ in one or more fea-tures; otherwise, one species will outcompete the other andthe extinction of the second species will inevitably result, aprocess referred to as competitive exclusion.

If resources are limiting, no two species can occupy thesame niche indefinitely without competition driving oneto extinction.

Chapter 25 Community Ecology 517

•

•••

••••

••• • ••

••

•

•

•••

••

•

••••• •

•••

••

•

••• ••

•••

•

• •••• • •• •

•••• • • • • • •

•• • •

•

••

•

200

150

100

50

00 0

04 8 12 16 20 24 20161284

Days Days(b) (c)

P. caudatum

P. aurelia

P. caudatum

P. bursaria

Pop

ulat

ion

dens

ity(m

easu

red

by v

olum

e)

50

75

25

FIGURE 25.3Competitive exclusionamong three species ofParamecium. In themicroscopic world,Paramecium is a ferociouspredator. Paramecia eat byingesting their prey; theircell membranes surroundbacterial or yeast cells,forming a food vacuolecontaining the prey cell.(a) In his experiments, Gause found that three speciesof Paramecium grew well alone in culture tubes. (b)But Paramecium caudatum would decline to extinctionwhen grown with P. aurelia because they shared thesame realized niche, and P. aurelia outcompeted P.caudatum for food resources. (c) However, P. caudatumand P. bursaria were able to coexist because the twohave different realized niches and thus avoidedcompetition.

0

50

100

150

200

50

100

150

200

50

100

150

200

Days

Pop

ulat

ion

dens

ity(m

easu

red

by v

olum

e)

40 8 12 16 20 24 40 8 12 16 20 24 40 8 12 16 20 24

0

Days

0

Days

P. aurelia P. caudatum

P. bursaria

••••

•

•• • •• • •

• ••••••

•••• •

•••

•• •• ••

• •• •• •

• • ••• •• •

•

(a)

Resource Partitioning Gause’s exclusion principle has a very important conse-quence: persistent competition between two species is rarein natural communities. Either one species drives the otherto extinction, or natural selection reduces the competitionbetween them. When the late Princeton ecologist RobertMacArthur studied five species of warblers, small insect-eating forest songbirds, he found that they all appeared tobe competing for the same resources. However, when hestudied them more carefully, he found that each species ac-tually fed in a different part of spruce trees and so ate dif-ferent subsets of insects. One species fed on insects nearthe tips of branches, a second within the dense foliage, athird on the lower branches, a fourth high on the trees anda fifth at the very apex of the trees. Thus, each species ofwarbler had evolved so as to utilize a different portion ofthe spruce tree resource. They subdivided the niche, parti-tioning the available resource so as to avoid direct competi-tion with one another.

Resource partitioning is often seen in similar speciesthat occupy the same geographical area. Such sympatricspecies often avoid competition by living in different por-tions of the habitat or by utilizing different food or otherresources (figure 25.4). This pattern of resource partition-ing is thought to result from the process of natural selec-tion causing initially similar species to diverge in resourceuse in order to reduce competitive pressures.

Evidence for the role of evolution comes from compari-son of species whose ranges are only partially overlapping.Where the two species co-occur, they tend to exhibitgreater differences in morphology (the form and structureof an organism) and resource use than do their allopatric

populations. Called character displacement, the differ-ences evident between sympatric species are thought tohave been favored by natural selection as a mechanism tofacilitate habitat partitioning and thus reduce competition.Thus, the two Darwin’s finches in figure 25.5 have bills ofsimilar size where the finches are allopatric, each living onan island where the other does not occur. On islands wherethey are sympatric, the two species have evolved beaks ofdifferent sizes, one adapted to larger seeds, the other tosmaller ones.

Sympatric species partition available resources,reducing competition between them.


50

25

050

25

050

25

07 9 11 13 15

Finch beak depth (mm)

Los HermanosIslets

Daphne MajorIsland

San Cristobal andSanta Maria Islands

G. fuliginosaAllopatric

G. fortisAllopatric

G. fuliginosaandG.

fortis Sympatric

�

Indi

vidu

als

in e

ach

size

cla

ss (

%)

FIGURE 25.5Character displacement in Darwin’s finches. These two speciesof finches (genus Geospiza) have bills of similar size whenallopatric, but different size when sympatric.

FIGURE 25.4Resource partitioning among sympatric lizard species. Species of Anolis lizards on Caribbean islands partition their tree habitats in avariety of ways. Some species of anoles occupy the canopy of trees (a), others use twigs on the periphery (b), and still others are found atthe base of the trunk (c). In addition, some use grassy areas in the open (d). When two species occupy the same part of the tree, they eitherutilize different-sized insects as food or partition the thermal microhabitat; for example, one might only be found in the shade, whereas theother would only bask in the sun. Most interestingly, the same pattern of resource partitioning has evolved independently on differentCaribbean islands.

(c)

(d)

(a)

(b)

Detecting Interspecific CompetitionIt is not simple to determine when two species are compet-ing. The fact that two species use the same resources neednot imply competition if that resource is not in limited sup-ply. If the population sizes of two species are negativelycorrelated, such that where one species has a large popula-tion, the other species has a small population and viceversa, the two species need not be competing for the samelimiting resource. Instead, the two species might be inde-pendently responding to the same feature of the environ-ment—perhaps one species thrives best in warm conditionsand the other in cool conditions.

Experimental Studies of Competition

Some of the best evidence for the existence of competi-tion comes from experimental field studies. By setting upexperiments in which two species either occur alone ortogether, scientists can determine whether the presenceof one species has a negative effect on a population of asecond species. For example, a variety of seed-eating ro-dents occur in the Chihuahuan Desert of the southwest-ern part of North America. In 1988, researchers set up aseries of 50 meter � 50 meter enclosures to investigatethe effect of kangaroo rats on other, smaller seed-eatingrodents. Kangaroo rats were removed from half of theenclosures, but not from the other enclosures. The wallsof all of the enclosures had holes in them that allowed ro-dents to come and go, but in the kangaroo rat removalplots, the holes were too small to allow the kangaroo ratsto enter. Over the course of the next three years, the re-searchers monitored the number of the other, smallerseed-eating rodents present in the plots. As figure 25.6 il-lustrates, the number of other rodents was substantiallyhigher in the absence of kangaroo rats, indicating thatkangaroo rats compete with the other rodents and limittheir population sizes.

A great number of similar experiments have indicatedthat interspecific competition occurs between many speciesof plants and animals. Effects of competition can be seen inaspects of population biology other than population size,such as behavior and individual growth rates. For example,two species of Anolis lizards occur on the island of St.Maarten. When one of the species, A. gingivinus, is placedin 12 m � 12 m enclosures without the other species, indi-vidual lizards grow faster and perch lower than lizards ofthe same species do when placed in enclosures in which A.pogus is also present.

Caution Is Necessary

Although experimental studies can be a powerful means ofunderstanding the interactions that occur between coexist-ing species, they have their limitations.

First, care is necessary in interpreting the results of fieldexperiments. Negative effects of one species on another do

not automatically indicate the existence of competition. Forexample, many similar-sized fish have a negative effect oneach other, but it results not from competition, but fromthe fact that adults of each species will prey on juveniles ofthe other species. In addition, the presence of one speciesmay attract predators, which then also prey on the secondspecies. In this case, the second species may have a lowerpopulation size in the presence of the first species due tothe presence of predators, even if they are not competing atall. Thus, experimental studies are most effective whenthey are combined with detailed examination of the ecolog-ical mechanism causing the negative effect of one specieson another species.

In addition, experimental studies are not always feasible.For example, the coyote has increased its population in theUnited States in recent years simultaneously with the de-cline of the grey wolf. Is this trend an indication that thespecies compete? Because of the size of the animals and thelarge geographic areas occupied by each individual, manip-ulative experiments involving fenced areas with only one orboth species—with each experimental treatment replicatedseveral times for statistical analysis—are not practical. Sim-ilarly, studies of slow-growing trees might require manycenturies to detect competition between adult trees. Insuch cases, detailed studies of the ecological requirementsof the species are our best bet to understanding interspe-cific interactions.

Experimental studies can provide strong tests of thehypothesis that interspecific competition occurs, butsuch studies have limitations. Detailed ecologicalstudies are important regardless of whetherexperiments are conducted.


10

15

5

0

199019891988

•

• ••

•

•

•

•

•• •

••

••

•

••

••

••

••

Kangaroo rats removedKangaroo rats present

Num

ber

ofca

ptur

esof

othe

rro

dent

s

FIGURE 25.6Detecting interspecific competition. This experiment tests theeffect of removal of kangaroo rats on the population size of otherrodents. Immediately after kangaroo rats were removed, thenumber of rodents increased relative to the enclosures that stillhad kangaroo rats. Notice that population sizes (as estimated bynumber of captures) increased and decreased in synchrony in thetwo treatments, probably reflecting changes in the weather.

Predation is the consuming of one organism by another.In this sense, predation includes everything from a leopardcapturing and eating an antelope, to a deer grazing onspring grass. When experimental populations are set upunder simple laboratory conditions, the predator often ex-terminates its prey and then becomes extinct itself, havingnothing left to eat (figure 25.7). However, if refuges areprovided for the prey, its population will drop to low levelsbut not to extinction. Low prey population levels will thenprovide inadequate food for the predators, causing thepredator population to decrease. When this occurs, theprey population can recover.

Predation and Prey PopulationsIn nature, predators can often have large effects on preypopulations. Some of the most dramatic examples involvesituations in which humans have either added or elimi-nated predators from an area. For example, the elimina-tion of large carnivores from much of the eastern UnitedStates has led to population explosions of white-taileddeer, which strip the habitat of all edible plant life. Simi-larly, when sea otters were hunted to near extinction onthe western coast of the United States, sea urchin popula-tions exploded.

Conversely, the introduction of rats, dogs, and cats tomany islands around the world has led to the decimationof native faunas. Populations of Galápagos tortoises onseveral islands are endangered, for example, by intro-duced rats, dogs, and cats, which eat eggs and young tor-toises. Similarly, several species of birds and reptiles havebeen eradicated by rat predation from New Zealand andnow only occur on a few offshore islands that the ratshave not reached. In addition, on Stephens Island, nearNew Zealand, every individual of the now extinctStephen Island wren was killed by a single lighthousekeeper’s cat!

A classic example of the role predation can play in acommunity involves the introduction of prickly pear cac-tus to Australia in the nineteenth century. In the absenceof predators, the cactus spread rapidly, by 1925 occupy-ing 12 million hectares of rangeland in an impenetrablemorass of spines that made cattle ranching difficult. Tocontrol the cactus, a predator from its natural habitat inArgentina, the moth Cactoblastis cactorum, was introducedbeginning in 1926. By 1940, cactus populations had beendecimated, and it now generally occurs in smallpopulations.

Predation and Evolution

Predation provides strong selective pressures on prey popu-lations. Any feature that would decrease the probability of

capture should be strongly favored. In the next three pages,we discuss a number of defense mechanisms in plants andanimals. In turn, the evolution of such features will causenatural selection to favor counteradaptations in predatorpopulations. In this way, a coevolutionary arms race mayensue in which predators and prey are constantly evolvingbetter defenses and better means of circumventing thesedefenses.

One example comes from the fossil record of molluscsand gastropods and their predators. During the Mesozoicperiod (approximately 65 to 225 million years ago), newforms of predatory fish and crustaceans evolved that wereable to crush or tear open shells. As a result, a variety of de-fensive measures evolved in molluscs and gastropods, in-cluding thicker shells, spines, and shells too smooth forpredators to be able to grasp. In turn, these adaptationsmay have pressured predators to evolve ever more effectivepredatory adaptations and tactics.

Predation can have substantial effects on preypopulations. As a result prey species often evolvedefensive adaptations.



0 1 2 3 4 5

40

80

120

Num

ber

of in

divi

dual

s

Time (days)

Paramecium

Didinium

FIGURE 25.7Predator-prey in the microscopic world. When the predatoryDidinium is added to a Paramecium population, the numbers ofDidinium initially rise, while the numbers of Paramecium steadilyfall. When the Paramecium population is depleted, however, theDidinium individuals also die.

Plant Defenses againstHerbivoresPlants have evolved many mecha-nisms to defend themselves from her-bivores. The most obvious are mor-phological defenses: thorns, spines,and prickles play an important role indiscouraging browsers, and planthairs, especially those that have aglandular, sticky tip, deter insect her-bivores. Some plants, such as grasses,deposit silica in their leaves, bothstrengthening and protecting them-selves. If enough silica is present intheir cells, these plants are simply tootough to eat.

Chemical Defenses

Significant as these morphological adaptations are, thechemical defenses that occur so widely in plants are evenmore crucial. Best known and perhaps most important inthe defenses of plants against herbivores are secondarychemical compounds. These are distinguished from pri-mary compounds, which are regular components of themajor metabolic pathways, such as respiration. Manyplants, and apparently many algae as well, contain verystructurally diverse secondary compounds that are eithertoxic to most herbivores or disturb their metabolismgreatly, preventing, for example, the normal developmentof larval insects. Consequently, most herbivores tend toavoid the plants that possess these compounds.

The mustard family (Brassicaceae) is characterized by agroup of chemicals known as mustard oils. These are thesubstances that give the pungent aromas and tastes tosuch plants as mustard, cabbage, watercress, radish, andhorseradish. The same tastes we enjoy signal the presenceof chemicals that are toxic to many groups of insects. Sim-ilarly, plants of the milkweed family (Asclepiadaceae) andthe related dogbane family (Apocynaceae) produce amilky sap that deters herbivores from eating them. In ad-dition, these plants usually contain cardiac glycosides,molecules named for their drastic effect on heart functionin vertebrates.

The Evolutionary Response of Herbivores

Certain groups of herbivores are associated with each fam-ily or group of plants protected by a particular kind of sec-ondary compound. These herbivores are able to feed onthese plants without harm, often as their exclusive foodsource. For example, cabbage butterfly caterpillars (sub-family Pierinae) feed almost exclusively on plants of themustard and caper families, as well as on a few other smallfamilies of plants that also contain mustard oils (figure

25.8). Similarly, caterpillars of monarch butterflies andtheir relatives (subfamily Danainae) feed on plants of themilkweed and dogbane families. How do these animalsmanage to avoid the chemical defenses of the plants, andwhat are the evolutionary precursors and ecological conse-quences of such patterns of specialization?

We can offer a potential explanation for the evolutionof these particular patterns. Once the ability to manufac-ture mustard oils evolved in the ancestors of the caper andmustard families, the plants were protected for a timeagainst most or all herbivores that were feeding on otherplants in their area. At some point, certain groups of in-sects—for example, the cabbage butterflies—evolved theability to break down mustard oils and thus feed on theseplants without harming themselves. Having developedthis ability, the butterflies were able to use a new resourcewithout competing with other herbivores for it. Often, ingroups of insects such as cabbage butterflies, sense organshave evolved that are able to detect the secondary com-pounds that their food plants produce. Clearly, the rela-tionship that has formed between cabbage butterflies andthe plants of the mustard and caper families is an exampleof coevolution.

The members of many groups of plants are protectedfrom most herbivores by their secondary compounds.Once the members of a particular herbivore groupevolve the ability to feed on them, these herbivores gainaccess to a new resource, which they can exploitwithout competition from other herbivores.


(a) (b)

FIGURE 25.8Insect herbivores are well suited to their hosts. (a) The green caterpillars of the cabbagebutterfly, Pieris rapae, are camouflaged on the leaves of cabbage and other plants on whichthey feed. Although mustard oils protect these plants against most herbivores, the cabbagebutterfly caterpillars are able to break down the mustard oil compounds. (b) An adultcabbage butterfly.

Animal Defenses against PredatorsSome animals that feed on plants rich in secondary com-pounds receive an extra benefit. When the caterpillars ofmonarch butterflies feed on plants of the milkweed family,they do not break down the cardiac glycosides that protectthese plants from herbivores. Instead, the caterpillars con-centrate and store the cardiac glycosides in fat bodies; theythen pass them through the chrysalis stage to the adult andeven to the eggs of the next generation. The incorporationof cardiac glycosides thus protects all stages of the monarchlife cycle from predators. A bird that eats a monarch but-terfly quickly regurgitates it (figure 25.9) and in the futureavoids the conspicuous orange-and-black pattern that char-acterizes the adult monarch. Some birds, however, appearto have acquired the ability to tolerate the protective chem-icals. These birds eat the monarchs.

Defensive Coloration

Many insects that feed on milkweed plants are brightly col-ored; they advertise their poisonous nature using an eco-logical strategy known as warning coloration, or apose-matic coloration. Showy coloration is characteristic ofanimals that use poisons and stings to repel predators,while organisms that lack specific chemical defenses are sel-dom brightly colored. In fact, many have cryptic col-oration—color that blends with the surroundings and thushides the individual from predators (figure 25.10). Camou-flaged animals usually do not live together in groups be-cause a predator that discovers one individual gains a valu-able clue to the presence of others.

Chemical Defenses

Animals also manufacture and use a startling array of sub-stances to perform a variety of defensive functions. Bees,wasps, predatory bugs, scorpions, spiders, and many otherarthropods use chemicals to defend themselves and to killtheir prey. In addition, various chemical defenses haveevolved among marine animals and the vertebrates, includ-ing venomous snakes, lizards, fishes, and some birds. Thepoison-dart frogs of the family Dendrobatidae producetoxic alkaloids in the mucus that covers their brightly col-ored skin (figure 25.11). Some of these toxins are so power-ful that a few micrograms will kill a person if injected intothe bloodstream. More than 200 different alkaloids havebeen isolated from these frogs, and some are playing im-portant roles in neuromuscular research. There is an inten-sive investigation of marine animals, algae, and floweringplants for new drugs to fight cancer and other diseases, oras sources of antibiotics.

Animals defend themselves against predators withwarning coloration, camouflage, and chemical defensessuch as poisons and stings.


FIGURE 25.9A blue jay learns that monarch butterflies taste bad. (a) Thiscage-reared jay had never seen a monarch butterfly before it triedeating one. (b) The same jay regurgitated the butterfly a fewminutes later. This bird will probably avoid trying to capture allorange-and-black insects in the future.

(a) (b)

FIGURE 25.10Cryptic coloration. An inchworm caterpillar (Necophora quernaria)(hanging from the upper twig) closely resembles a twig.

FIGURE 25.11Vertebrate chemical defenses. Frogs of the familyDendrobatidae, abundant in the forests of Latin America, areextremely poisonous to vertebrates. Dendrobatids advertise theirtoxicity with aposematic coloration, as shown here.

MimicryDuring the course of their evolution, many species havecome to resemble distasteful ones that exhibit aposematiccoloration. The mimic gains an advantage by looking likethe distasteful model. Two types of mimicry have beenidentified: Batesian and Müllerian mimicry.

Batesian Mimicry

Batesian mimicry is named for Henry Bates, the Britishnaturalist who first brought this type of mimicry to gen-eral attention in 1857. In his journeys to the Amazon re-gion of South America, Bates discovered many instancesof palatable insects that resembled brightly colored, dis-tasteful species. He reasoned that the mimics would beavoided by predators, who would be fooled by the dis-guise into thinking the mimic actually is the distastefulmodel.

Many of the best-known examples of Batesian mimicryoccur among butterflies and moths. Obviously, predators insystems of this kind must use visual cues to hunt for theirprey; otherwise, similar color patterns would not matter topotential predators. There is also increasing evidence indi-cating that Batesian mimicry can also involve nonvisualcues, such as olfaction, although such examples are less ob-vious to humans.

The kinds of butterflies that provide the models in Bate-sian mimicry are, not surprisingly, members of groupswhose caterpillars feed on only one or a few closely relatedplant families. The plant families on which they feed arestrongly protected by toxic chemicals. The model butter-flies incorporate the poisonous molecules from these plantsinto their bodies. The mimic butterflies, in contrast, belongto groups in which the feeding habits of the caterpillars arenot so restricted. As caterpillars, these butterflies feed on anumber of different plant families unprotected by toxicchemicals.

One often-studied mimic among North American but-terflies is the viceroy, Limenitis archippus (figure 25.12a).This butterfly, which resembles the poisonous monarch,ranges from central Canada through much of the UnitedStates and into Mexico. The caterpillars feed on willowsand cottonwoods, and neither caterpillars nor adults werethought to be distasteful to birds, although recent findingsmay dispute this. Interestingly, the Batesian mimicry seenin the adult viceroy butterfly does not extend to the cater-pillars: viceroy caterpillars are camouflaged on leaves, re-sembling bird droppings, while the monarch’s distastefulcaterpillars are very conspicuous.

Müllerian Mimicry

Another kind of mimicry, Müllerian mimicry, was namedfor German biologist Fritz Müller, who first described it in1878. In Müllerian mimicry, several unrelated but pro-tected animal species come to resemble one another (figure

25.12b). If animals that resemble one another are all poiso-nous or dangerous, they gain an advantage because a preda-tor will learn more quickly to avoid them. In some cases,predator populations even evolve an innate avoidance ofspecies; such evolution may occur more quickly when mul-tiple dangerous prey look alike.

In both Batesian and Müllerian mimicry, mimic andmodel must not only look alike but also act alike if preda-tors are to be deceived. For example, the members of sev-eral families of insects that closely resemble wasps behavesurprisingly like the wasps they mimic, flying often and ac-tively from place to place.

In Batesian mimicry, unprotected species resembleothers that are distasteful. Both species exhibitaposematic coloration. In Müllerian mimicry, two ormore unrelated but protected species resemble oneanother, thus achieving a kind of group defense.


(a) Batesian mimicry: Monarch (Danaus) is poisonous; viceroy(Limenitis) is palatable mimic

(b) Müllerian mimicry: two pairs of mimics; all are distasteful

Heliconius erato Heliconius melpomene

Danaus plexippus Limenitis archippus

Heliconius sapho Heliconius cydno

FIGURE 25.12Mimicry. (a) Batesian mimicry. Monarch butterflies (Danausplexippus) are protected from birds and other predators by thecardiac glycosides they incorporate from the milkweeds anddogbanes they feed on as larvae. Adult monarch butterfliesadvertise their poisonous nature with warning coloration. Viceroybutterflies (Limenitis archippus) are Batesian mimics of thepoisonous monarch. (b) Pairs of Müllerian mimics. Heliconius eratoand H. melpomene are sympatric, and H. sapho and H. cydno aresympatric. All of these butterflies are distasteful. They haveevolved similar coloration patterns in sympatry to minimizepredation; predators need only learn one pattern to avoid.

Coevolution and SymbiosisThe plants, animals, protists, fungi, and bacteria that livetogether in communities have changed and adjusted toone another continually over a period of millions ofyears. For example, many features of flowering plantshave evolved in relation to the dispersal of the plant’s ga-metes by animals (figure 25.13). These animals, in turn,have evolved a number of special traits that enable themto obtain food or other resources efficiently from theplants they visit, often from their flowers. While doingso, the animals pick up pollen, which they may deposit onthe next plant they visit, or seeds, which may be left else-where in the environment, sometimes a great distancefrom the parental plant.

Such interactions, which involve the long-term, mutualevolutionary adjustment of the characteristics of the mem-bers of biological communities, are examples of coevolu-tion, a phenomenon we have already seen in predator-preyinteractions.

Symbiosis Is Widespread

Another type of coevolution involves symbiotic relation-ships in which two or more kinds of organisms live to-gether in often elaborate and more-or-less permanent re-lationships. All symbiotic relationships carry the potentialfor coevolution between the organisms involved, and inmany instances the results of this coevolution are fascinat-ing. Examples of symbiosis include lichens, which are asso-ciations of certain fungi with green algae or cyanobacte-ria. Lichens are discussed in more detail in chapter 36.Another important example are mycorrhizae, the associa-tion between fungi and the roots of most kinds of plants.The fungi expedite the plant’s absorption of certain nutri-ents, and the plants in turn provide the fungi with carbo-hydrates. Similarly, root nodules that occur in legumesand certain other kinds of plants contain bacteria that fixatmospheric nitrogen and make it available to their hostplants.

In the tropics, leafcutter ants are often so abundantthat they can remove a quarter or more of the total leafsurface of the plants in a given area. They do not eatthese leaves directly; rather, they take them to under-ground nests, where they chew them up and inoculatethem with the spores of particular fungi. These fungi arecultivated by the ants and brought from one speciallyprepared bed to another, where they grow and repro-duce. In turn, the fungi constitute the primary food ofthe ants and their larvae. The relationship between leaf-cutter ants and these fungi is an excellent example ofsymbiosis.

Kinds of Symbiosis

The major kinds of symbiotic relationships include (1)commensalism, in which one species benefits while theother neither benefits nor is harmed; (2) mutualism, inwhich both participating species benefit; and (3) para-sitism, in which one species benefits but the other isharmed. Parasitism can also be viewed as a form of preda-tion, although the organism that is preyed upon does notnecessarily die.

Coevolution is a term that describes the long-termevolutionary adjustments of species to one another. Insymbiosis two or more species interact closely, with atleast one species benefitting.



FIGURE 25.13Pollination by bat. Many flowers have coevolved with otherspecies to facilitate pollen transfer. Insects are widely known aspollinators, but they’re not the only ones. Notice the cargo ofpollen on the bat’s snout.

CommensalismCommensalism is a symbiotic rela-tionship that benefits one speciesand neither hurts nor helps theother. In nature, individuals of onespecies are often physically attachedto members of another. For example,epiphytes are plants that grow on thebranches of other plants. In general,the host plant is unharmed, while theepiphyte that grows on it benefits.Similarly, various marine animals,such as barnacles, grow on other,often actively moving sea animalslike whales and thus are carried pas-sively from place to place. These“passengers” presumably gain moreprotection from predation than theywould if they were fixed in one place,and they also reach new sources offood. The increased water circulationthat such animals receive as theirhost moves around may be of greatimportance, particularly if the pas-sengers are filter feeders. The ga-metes of the passenger are also morewidely dispersed than would be thecase otherwise.

Examples of Commensalism

The best-known examples of commensalism involve the re-lationships between certain small tropical fishes and seaanemones, marine animals that have stinging tentacles (seechapter 44). These fish have evolved the ability to liveamong the tentacles of sea anemones, even though thesetentacles would quickly paralyze other fishes that touchedthem (figure 25.14). The anemone fishes feed on the detri-tus left from the meals of the host anemone, remaining un-injured under remarkable circumstances.

On land, an analogous relationship exists between birdscalled oxpeckers and grazing animals such as cattle or rhi-noceros. The birds spend most of their time clinging to theanimals, picking off parasites and other insects, carryingout their entire life cycles in close association with the hostanimals.

When Is Commensalism Commensalism?

In each of these instances, it is difficult to be certainwhether the second partner receives a benefit or not;there is no clear-cut boundary between commensalism

and mutualism. For instance, it may be advantageous tothe sea anemone to have particles of food removed fromits tentacles; it may then be better able to catch otherprey. Similarly, while often thought of as commensalism,the association of grazing mammals and gleaning birds isactually an example of mutualism. The mammal benefitsby having parasites and other insects removed from itsbody, but the birds also benefit by gaining a dependablesource of food.

On the other hand, commensalism can easily transformitself into parasitism. For example, oxpeckers are alsoknown to pick not only parasites, but also scabs off theirgrazing hosts. Once the scab is picked, the birds drink theblood that flows from the wound. Occasionally, the cumu-lative effect of persistent attacks can greatly weaken theherbivore, particularly when conditions are not favorable,such as during droughts.

Commensalism is the benign use of one organism byanother.


FIGURE 25.14Commensalism in the sea. Clownfishes, such as this Amphiprion perideraion in Guam,often form symbiotic associations with sea anemones, gaining protection by remainingamong their tentacles and gleaning scraps from their food. Different species of anemonessecrete different chemical mediators; these attract particular species of fishes and may betoxic to the fish species that occur symbiotically with other species of anemones in the samehabitat. There are 26 species of clownfishes, all found only in association with seaanemones; 10 species of anemones are involved in such associations, so that some of theanemone species are host to more than one species of clownfish.

MutualismMutualism is a symbiotic relationship among organisms inwhich both species benefit. Examples of mutualism are offundamental importance in determining the structure of bi-ological communities. Some of the most spectacular exam-ples of mutualism occur among flowering plants and theiranimal visitors, including insects, birds, and bats. As we willsee in chapter 37, during the course of their evolution, thecharacteristics of flowers have evolved in large part in rela-tion to the characteristics of the animals that visit them forfood and, in doing so, spread their pollen from individualto individual. At the same time, characteristics of the ani-mals have changed, increasing their specialization for ob-taining food or other substances from particular kinds offlowers.

Another example of mutualism involves ants and aphids.Aphids, also called greenflies, are small insects that suckfluids from the phloem of living plants with their piercingmouthparts. They extract a certain amount of the sucroseand other nutrients from this fluid, but they excrete muchof it in an altered form through their anus. Certain antshave taken advantage of this—in effect, domesticating theaphids. The ants carry the aphids to new plants, where theycome into contact with new sources of food, and then con-sume as food the “honeydew” that the aphids excrete.

Ants and Acacias

A particularly striking example of mutualism involves antsand certain Latin American species of the plant genus Aca-cia. In these species, certain leaf parts, called stipules, aremodified as paired, hollow thorns. The thorns are inhab-ited by stinging ants of the genus Pseudomyrmex, which donot nest anywhere else (figure 25.15). Like all thorns thatoccur on plants, the acacia horns serve to deter herbivores.

At the tip of the leaflets of these acacias are unique, pro-tein-rich bodies called Beltian bodies, named after thenineteenth-century British naturalist Thomas Belt. Beltianbodies do not occur in species of Acacia that are not inhab-ited by ants, and their role is clear: they serve as a primaryfood for the ants. In addition, the plants secrete nectarfrom glands near the bases of their leaves. The ants con-sume this nectar as well, feeding it and the Beltian bodiesto their larvae.

Obviously, this association is beneficial to the ants, andone can readily see why they inhabit acacias of this group.The ants and their larvae are protected within the swollenthorns, and the trees provide a balanced diet, including thesugar-rich nectar and the protein-rich Beltian bodies.What, if anything, do the ants do for the plants?

Whenever any herbivore lands on the branches or leavesof an acacia inhabited by ants, the ants, which continuallypatrol the acacia’s branches, immediately attack and devourthe herbivore. The ants that live in the acacias also helptheir hosts to compete with other plants. The ants cut away

any branches of other plants that touch the acacia in whichthey are living. They create, in effect, a tunnel of lightthrough which the acacia can grow, even in the lush decid-uous forests of lowland Central America. In fact, when anant colony is experimentally removed from a tree, the aca-cia is unable to compete successfully in this habitat. Finally,the ants bring organic material into their nests. The partsthey do not consume, together with their excretions, pro-vide the acacias with an abundant source of nitrogen.

As with commensalism, however, things are not alwaysas they seem. Ant-acacia mutualisms also occur in Africa. InKenya, several species of acacia ants occur, but only onespecies occurs on any tree. One species, Crematogaster ni-griceps, is competitively inferior to two of the other species.To prevent invasion by other ant species, C. nigricepsprunes the branches of the acacia, preventing it from com-ing into contact with branches of other trees, which wouldserve as a bridge for invaders. Although this behavior isbeneficial to the ant, it is detrimental to the tree, as it de-stroys the tissue from which flowers are produced, essen-tially sterilizing the tree. In this case, what has initiallyevolved as a mutualistic interaction has instead become aparasitic one.

Mutualism involves cooperation between species, to themutual benefit of both.


FIGURE 25.15Mutualism: ants and acacias. Ants of the genus Pseudomyrmexlive within the hollow thorns of certain species of acacia trees inLatin America. The nectaries at the bases of the leaves and theBeltian bodies at the ends of the leaflets provide food for the ants.The ants, in turn, supply the acacias with organic nutrients andprotect the acacia from herbivores.

Parasitism Parasitism may be regarded as a special form of symbiosisin which the predator, or parasite, is much smaller than theprey and remains closely associated with it. Parasitism isharmful to the prey organism and beneficial to the parasite.The concept of parasitism seems obvious, but individual in-stances are often surprisingly difficult to distinguish frompredation and from other kinds of symbiosis.

External Parasites

Parasites that feed on the exterior surface of an organismare external parasites, or ectoparasites. Many instances ofexternal parasitism are known (figure 25.16). Lice, whichlive on the bodies of vertebrates—mainly birds and mam-mals—are normally considered parasites. Mosquitoes arenot considered parasites, even though they draw food frombirds and mammals in a similar manner to lice, becausetheir interaction with their host is so brief.

Parasitoids are insects that lay eggs on living hosts.This behavior is common among wasps, whose larvae feedon the body of the unfortunate host, often killing it.

Internal Parasites

Vertebrates are parasitized internally by endoparasites,members of many different phyla of animals and protists.Invertebrates also have many kinds of parasites that livewithin their bodies. Bacteria and viruses are not usuallyconsidered parasites, even though they fit our definitionprecisely.

Internal parasitism is generally marked by much moreextreme specialization than external parasitism, as shownby the many protist and invertebrate parasites that infecthumans. The more closely the life of the parasite is linkedwith that of its host, the more its morphology and behaviorare likely to have been modified during the course of itsevolution. The same is true of symbiotic relationships of allsorts. Conditions within the body of an organism are dif-ferent from those encountered outside and are apt to bemuch more constant. Consequently, the structure of an in-ternal parasite is often simplified, and unnecessary arma-ments and structures are lost as it evolves.

Brood Parasitism

Not all parasites consume the body of their host. In broodparasitism, birds like cowbirds and European cuckoos laytheir eggs in the nests of other species. The host parentsraise the brood parasite as if it were one of their ownclutch, in many cases investing more in feeding the im-poster than in feeding their own offspring (figure 25.17).The brood parasite reduces the reproductive success of thefoster parent hosts, so it is not surprising that in some casesnatural selection has fostered the hosts’ ability to detect

parasite eggs and reject them. What is more surprising isthat in many other species, the ability to detect parasiteeggs has not evolved.

In parasitism, one organism serves as a host to anotherorganism, usually to the host’s disadvantage.


FIGURE 25.16An external parasite. The flowering plant dodder (Cuscuta) is aparasite and has lost its chlorophyll and its leaves in the course ofits evolution. Because it is heterotrophic, unable to manufactureits own food, dodder obtains its food from the host plants itgrows on.

FIGURE 25.17Brood parasitism. This bird is feeding a cuckoo chick in its nest.The cuckoo chick is larger than the adult bird, but the bird doesnot recognize that the cuckoo is not its own offspring. Cuckoomothers sneak into the nests of other birds and lay an egg,entrusting the care of their offspring to an unwitting bird ofanother species.

Interactions among EcologicalProcessesWe have seen the different ways in which species within acommunity can interact with each other. In nature, how-ever, more than one type of interaction usually occurs atthe same time. In many cases, the outcome of one type ofinteraction is modified or even reversed when another typeof interaction is also occurring.

Predation Reduces Competition

When resources are limiting, a superior competitor caneliminate other species from a community. However,predators can prevent or greatly reduce competitive ex-clusion by reducing the numbers of individuals of compet-ing species. A given predator may often feed on two,three, or more kinds of plants or animals in a given com-munity. The predator’s choice depends partly on the rela-tive abundance of the prey options. In other words, apredator may feed on species A when it is abundant andthen switch to species B when A is rare. Similarly, a givenprey species may be a primary source of food for increas-ing numbers of species as it becomes more abundant. Inthis way, superior competitors may be prevented fromoutcompeting other species.

Such patterns are often characteristic of biologicalcommunities in marine intertidal habitats. For example,in preying selectively on bivalves, sea stars prevent bi-valves from monopolizing such habitats, opening upspace for many other organisms (figure 25.18). When seastars are removed from a habitat, species diversity fallsprecipitously, the seafloor community coming to be dom-inated by a few species of bivalves. Because predationtends to reduce competition in natural communities, it isusually a mistake to attempt to eliminate a major preda-tor such as wolves or mountain lions from a community.The result is to decrease rather than increase the biologi-cal diversity of the community, the opposite of what isintended.

Parasitism May Counter Competition

Parasites may effect sympatric species differently and thusinfluence the outcome of interspecific interactions. In aclassic experiment, Thomas Park of the University ofChicago investigated interactions between two flour bee-tles, Tribolium castaneum and T. confusum with a parasite,Adelina. In the absence of the parasite, T. castaneum is dom-inant and T. confusum normally goes extinct. When the par-asite is present, however, the outcome is reversed and T.castaneum perishes. Similar effects of parasites in naturalsystems have been observed in many species. For example,in the Anolis lizards of St. Maarten mentioned previously,the competitively inferior species is resistant to malaria,whereas the other species is highly susceptible. Only in

areas in which the malaria parasite occurs are the twospecies capable of coexisting.

Indirect Effects

In some cases, species may not directly interact, yet thepresence of one species may effect a second species by wayof interactions with a third species. Such effects are termedindirect effects. For example, in the Chihuahuan Desert,rodents and ants both eat seeds. Thus, one might expectthem to compete with each other. However, when all ro-dents were completely removed from large enclosures (un-like the experiment discussed above, there were no holes in


FIGURE 25.18Predation reduces competition. (a) In a controlled experimentin a coastal ecosystem, an investigator removed a key predator(Pisaster). (b) In response, fiercely competitive mussels exploded ingrowth, effectively crowding out seven other indigenous species.

(a)

(b)

the enclosure walls, so once removed, rodents couldn’t getback in), ant populations first increased, but then declined(figure 25.19). The initial increase was the expected resultof removing a competitor; why did it reverse? The answerreveals the intricacies of natural ecosystems (figure 25.20).Rodents prefer large seeds, whereas ants prefer smallerseeds. Further, in this system plants with large seeds arecompetitively superior to plants with small seeds. Thus, theremoval of rodents leads to an increase in the number ofplants with large seeds, which reduces the number of smallseeds available to ants, which thus leads to a decline in antpopulations. Thus, the effect of rodents on ants is compli-cated: a direct negative effect of resource competition andan indirect, positive effect mediated by plant competition.

Keystone Species

Species that have particularly strong effects on the compo-sition of communities are termed keystone species.Predators, such as the starfish, can often serve as keystonespecies by preventing one species from outcompeting oth-ers, thus maintaining high levels of species richness in acommunity.

There are, however, a wide variety of other types of key-stone species. Some species manipulate the environment inways that create new habitats for other species. Beavers, forexample, change running streams into small impound-ments, changing the flow of water and flooding areas (fig-ure 25.21). Similarly, alligators excavate deep holes at thebottoms of lakes. In times of drought, these holes are theonly areas in which water remains, thus allowing aquaticspecies that otherwise would perish to persist until thedrought ends and the lake refills.

Many different processes are likely to be occurringsimultaneously within communities. Only byunderstanding how these processes interact will we beable to understand how communities function.


•

••

••

••

•

•

•

• • •

•

•• •

••

•

••

• •• •

60

40

20

Sampling periods

Num

ber

ofan

tcol

onie

s

Oct 74 May 75 Sep 75 May 76 Aug 76 Jul 77

Rodents removedRodents not removed

FIGURE 25.19Change in ant population size after the removal of rodents.Ants initially increased in population size relative to ants in theenclosures from which rodents weren’t removed, but then theseant populations declined.

Rodents

Large seeds Small seeds

Ants

–

–

–

+ +

FIGURE 25.20Rodent-ant interactions. Rodents and ants both eat seeds, so thepresence of rodents has a negative effect on ants and vice versa.However, the presence of rodents has a negative effect on largeseeds. In turn, the number of plants with large seeds has a negativeeffect on plants that produce small seeds. Hence, the presence ofrodents should increase the number of small seeds. In turn, thenumber of small seeds has a positive effect on ant populations.Thus, indirectly, the presence of rodents has a positive effect onant population size.

FIGURE 25.21Example of a keystone species. Beavers, by constructing damsand transforming flowing streams into ponds, create new habitatsfor many plant and animal species.

Even when the climate of an area remains stable year afteryear, ecosystems have a tendency to change from simple tocomplex in a process known as succession. This process isfamiliar to anyone who has seen a vacant lot or clearedwoods slowly become occupied by an increasing number ofplants, or a pond become dry land as it is filled with vegeta-tion encroaching from the sides.

SuccessionIf a wooded area is cleared and left alone, plants willslowly reclaim the area. Eventually, traces of the clearingwill disappear and the area will again be woods. This kindof succession, which occurs in areas where an existingcommunity has been disturbed, is called secondarysuccession.

In contrast, primary succession occurs on bare, lifelesssubstrate, such as rocks, or in open water, where organismsgradually move into an area and change its nature. Primarysuccession occurs in lakes left behind after the retreat ofglaciers, on volcanic islands that rise above the sea, and onland exposed by retreating glaciers (figure 25.22). Primarysuccession on glacial moraines provides an example (figure25.23). On bare, mineral-poor soil, lichens grow first,forming small pockets of soil. Acidic secretions from thelichens help to break down the substrate and add to the ac-cumulation of soil. Mosses then colonize these pockets ofsoil, eventually building up enough nutrients in the soil foralder shrubs to take hold. Over a hundred years, the aldersbuild up the soil nitrogen levels until spruce are able tothrive, eventually crowding out the alder and forming adense spruce forest.

In a similar example, an oligotrophic lake—one poor innutrients—may gradually, by the accumulation of organicmatter, become eutrophic—rich in nutrients. As this oc-curs, the composition of communities will change, first in-creasing in species richness and then declining.

Primary succession in different habitats often eventuallyarrives at the same kinds of vegetation—vegetation charac-teristic of the region as a whole. This relationship ledAmerican ecologist F. E. Clements, at about the turn of thecentury, to propose the concept of a final climax commu-nity. With an increasing realization that (1) the climatekeeps changing, (2) the process of succession is often veryslow, and (3) the nature of a region’s vegetation is being de-termined to an increasing extent by human activities, ecol-ogists do not consider the concept of “climax community”to be as useful as they once did.

Why Succession Happens

Succession happens because species alter the habitat andthe resources available in it in ways that favor other species.

Three dynamic concepts are of critical importance in theprocess: tolerance, inhibition, and facilitation.

1. Tolerance. Early successional stages are character-ized by weedy r-selected species that are tolerant ofthe harsh, abiotic conditions in barren areas.

2. Facilitation. The weedy early successional stages in-troduce local changes in the habitat that favor other,less weedy species. Thus, the mosses in the GlacierBay succession convert nitrogen to a form that allowsalders to invade. The alders in turn lower soil pH astheir fallen leaves decompose, and spruce and hem-lock, which require acidic soil, are able to invade.

3. Inhibition. Sometimes the changes in the habitatcaused by one species, while favoring other species,inhibit the growth of the species that caused them.Alders, for example, do not grow as well in acidic soilas the spruce and hemlock that replace them.

Over the course of succession, the number of speciestypically increases as the environment becomes more hos-pitable. In some cases, however, as ecosystems mature,more K-selected species replace r-selected ones, and supe-rior competitors force out other species, leading ultimatelyto a decline in species richness.

Communities evolve to have greater total biomass andspecies richness in a process called succession.


25.4 Ecological succession may increase species richness.

Pioneer mosses Invadingalders

Alderthickets

Spruceforest

Year 1 Year 100 Year 200

50

100

150

200

250

300

Nitr

ogen

con

cent

ratio

n(g

/m2

of s

urfa

ce)

b

c

Nitrogenin mineral soil

Nitrogenin forest floor

FIGURE 25.22Plant succession produces progressive changes in the soil.Initially, the glacial moraine at Glacier Bay, Alaska, portrayed infigure 25.23, had little soil nitrogen, but nitrogen-fixing alders ledto a buildup of nitrogen in the soil, encouraging the subsequentgrowth of the conifer forest. Letters in the graph correspond tophotographs in parts b and c of figure 25.23.

The Role of DisturbanceDisturbances often interrupt the succession of plant com-munities. Depending on the magnitude of the disturbance,communities may revert to earlier stages of succession oreven, in extreme cases, begin at the earliest stages of pri-mary succession. Disturbances severe enough to disruptsuccession include calamities such as forest fires, drought,and floods. Animals may also cause severe disruptions.Gypsy moths can devastate a forest by consuming its trees.Unregulated deer populations may grow explosively, thedeer overgrazing and so destroying the forest they live in,in the same way too many cattle overgraze a pasture by eat-ing all available grass down to the ground.

Intermediate Disturbance Hypothesis

In some cases, disturbance may act to increase the speciesrichness of an area. According to the intermediate disturbancehypothesis, communities experiencing moderate amounts ofdisturbance will have higher levels of species richness thancommunities experiencing either little or great amounts ofdisturbance. Two factors could account for this pattern. First,in communities in which moderate amounts of disturbanceoccur, patches of habitat will exist at different successionalstages. Thus, within the area as a whole, species diversity willbe greatest because the full range of species—those character-istic of all stages of succession—will be present. For example,a pattern of intermittent episodic disturbance that producesgaps in the rain forest (like when a tree falls) allows invasionof the gap by other species (figure 25.24). Eventually, thespecies inhabiting the gap will go through a successional se-quence, one tree replacing another, until a canopy tree speciescomes again to occupy the gap. But if there are lots of gaps ofdifferent ages in the forest, many different species will coexist,some in young gaps, others in older ones.

Second, moderate levels of disturbance may preventcommunities from reaching the final stages of succession,

in which a few dominant competitors eliminate most of theother species. On the other hand, too much disturbancemight leave the community continually in the earlieststages of succession, when species richness is relatively low.

Ecologists are increasingly realizing that disturbance isthe norm, rather than the exception, in many communities.As a result, the idea that communities inexorably movealong a successional trajectory culminating in the develop-ment of a climax community is no longer widely accepted.Rather, predicting the state of a community in the futuremay be difficult because the unpredictable occurrence ofdisturbances will often counter successional changes. Un-derstanding the role that disturbances play in structuringcommunities is currently an important area of investigationin ecology.

Succession is often disrupted by natural or humancauses. In some cases, intermediate levels ofdisturbance may maximize the species richness of acommunity.


(a) (b) (c)

FIGURE 25.23Primary succession at Alaska’s Glacier Bay. (a) The sides of the glacier have been retreating at a rate of some 8 meters a year, leavingbehind exposed soil from which nitrogen and other minerals have been leached out. The first invaders of these exposed sites are pioneermoss species with nitrogen-fixing mutualistic microbes. Within 20 years, young alder shrubs take hold. (b) Rapidly fixing nitrogen, theysoon form dense thickets. As soil nitrogen levels rise, (c) spruce crowd out the mature alders, forming a forest.

FIGURE 25.24Intermediatedisturbance. Asingle fallen treecreates a smalllight gap in thetropical rain forestof Panama. Suchgaps play a keyrole in maintainingthe high speciesdiversity of therain forest.


Chapter 25 Summary Questions Media Resources

25.1 Interactions among competing species shape ecological niches.

• Each species plays a specific role in its ecosystem; thisrole is called its niche.

• An organism’s fundamental niche is the total nichethat the organism would occupy in the absence ofcompetition. Its realized niche is the actual niche itoccupies in nature.

• Two species cannot occupy the same niche for long ifresources are limiting; one will outcompete the other,driving it to extinction.

• Species can coexist by partitioning resources to mini-mize competition.

1. What is the differencebetween interspecificcompetition and intraspecificcompetition? What is Gause’sprinciple of competitiveexclusion?2. Is the term niche synonymouswith the term habitat? Why orwhy not? How does anorganism’s fundamental nichediffer from its realized niche?

• Plants are often protected from herbivores bychemicals they manufacture.

• Warning, or aposematic, coloration is characteristicof organisms that are poisonous, sting, or areotherwise harmful. In contrast, cryptic coloration, orcamouflage, is characteristic of nonpoisonousorganisms.

• Predator-prey relationships are of crucial importancein limiting population sizes in nature.

3. What morphological defensesdo plants use to defendthemselves against herbivores?4. Consider aposematiccoloration, cryptic coloration,and Batesian mimicry. Whichwould be associated with anadult viceroy butterfly? Whichwould be associated with a larvalmonarch butterfly? Whichwould be associated with a larvalviceroy butterfly?


• Coevolution occurs when different kinds oforganisms evolve adjustments to one another overlong periods of time.

• Many organisms have coevolved to a point ofdependence. In mutualism the relationship ismutually beneficial; in commensalism, only oneorganism benefits while the other is unharmed; andin parasitism one organism serves as a host toanother, usually to the host’s disadvantage.

5. Why is eliminating predatorsa bad idea for species richness?6. How can predation andcompetition interact inregulating species diversity of acommunity?


• Primary succession takes place in barren areas, likerocks or open water. Secondary succession takes placein areas where the original communities of organismshave been disturbed.

• Succession occurs because of tolerance, facilitation,and inhibition.

• Disturbance can disrupt successional changes. Insome cases, disturbance can increase species richnessof a community.

7. Why have scientists alteredthe concept of a final, climaxvegetation in a given ecosystem?What types of organisms areoften associated with early stagesof succession? What is the roleof disturbance in succession?

25.4 Ecological succession may increase species richness.

www.mhhe.com/raven6e www.biocourse.com

• Introduction toCommunities

• CommunityOrganization

• On Science Article:Killer Bees

• Student Research:Hermit Crab—SeaAnemone Associations

• Succession

• Book Review: Guns,Germs, and Steel byDiamond

Documents

Community Ecology - Seneca Valley School Districtcompetition driving one to extinction. Resource Partitioning. Species that live together partition the available resources, reducing